1. Field of the Invention
This invention relates to the art of firefighting equipment.
More particularly, the invention relates to a system for controlling the pressure output of a fire pump.
In a further and more specific aspect, the instant invention concerns an electronically operated system for controlling the pressure output of an engine-driven fire pump.
2. Description of the Prior Art
Controlling the discharge pressure of an engine-driven fire pump mounted on a fire truck is vital. The pump must supply water at a continuous rate and steady pressure so that firemen operating hand nozzles at a fire scene can control the reaction force generated by their nozzles. This is no simple matter, however, since even slight variations in pressure in the supply line leading to the intake of the pump are amplified by the pump on the discharge side, causing surges or oscillations in the water flow discharge at the nozzle and corresponding changes in the reaction forces. Such changes are extremely dangerous, as they can pull a nozzle out of a fireman's grip, or even throw him off a ladder or a ledge.
The simplest prior art device for controlling the pressure output of a fire pump is a mechanical relief valve which opens to discharge excess water when the incoming pressure is higher than the desired output pressure. A shortcoming of such a valve, however, is that, because the relief point must be manually set, it requires the use of a human operator, and can not be used with robotically controlled equipment of the type which is increasingly being used at high-intensity, high-risk fires. Furthermore, the relief valve only functions to dissipate excess incoming pressure, and has no utility in situations where the incoming pressure is too low, such as when the water source is being depleted or another hose is connected to the system. In addition, if the pump engine continues to operate at full speed after the relief valve is opened, water will be continuously dumped from the system, resulting in needless waste, as well as flooding of the area where the fire truck is located.
More recently, electronically operated pressure control systems have been introduced. Two such systems are disclosed in U.S. Pat. Nos. 3,786,869 and 4,189,005 to McLoughlin, the subject matter of which is herein incorporated by reference. In these systems, the desired output pressure is dialed in or otherwise transmitted to a control box on board the fire truck, where it is compared to the actual output pressure as measured by a transducer. Any difference between the desired and actual output pressures is converted to an electrical signal which is fed to a DC motor which increases or decreases the RPM of the centrifugal pump as needed until the desired output pressure is reached. A shortcoming of this type of system is that, because the response time of the servo-mechanism controlling the engine is slow, much time can pass before the appropriate RPM and correct discharge pressure are reached. This is especially troublesome during transient events, such as overpressure spikes, where the system's response time is greater than the length of the event. Furthermore, no allowance is made for situations such as when the engine is already at idle and the incoming pressure suddenly increases, or is higher than desired, such as can happen when the pump is connected to a hydrant.
Another pressure control system of interest is disclosed in German Patent No. 1,274,402 to Muller & Co., which discloses an engine-driven pump which responds to an overpressure in the supply line by simultaneously opening a pressure relief valve and mechanically reducing the engine speed. The shortcomings of this purely mechanical system are that by its nature, in cases of overpressure, the relief valve will always be open to some extent, allowing some fluid to always bypass the relief valve, and the engine RPM will always be above its ideal setting to a certain extent.
Accordingly, a need exists for a new and improved electronically operated fire pump discharge pressure control system for quickly and safely responding to drops or increases in the incoming pressure of a fire pump, which change the discharge pressure required, as well as changes in discharge pressure due to the opening or shutting of various valves downstream of the pump.